Most float level sensors include a shaft which has substantially circular cross-section on which moves a float with a circular hole in the center. In some float level sensors multiple floats move up and down at different positions on a shaft to determine different level ranges and sensors associated with the floats provide signals representing levels in the different ranges. In other instances a single float is provided with a vertical linear movement dependent upon the level range required to be monitored.
In an application where the liquids are dirty and corrosive such as, for example, waste tanks or holding tanks in boats, long range buses, recreational vehicles, airplanes, trains, etc., the surface of the shaft has to be specially coated with a non-slip finish to prevent deposits forming on the shaft thus interfering with the movement of the float on the shaft. In the past one of the ways to achieve a non-slip surface has been by the application of polytetrafluoroethylene polymer, or PTFE for short. An example of this product is sold under the trade mark "TEFLON" and is well known as a material that has very low coefficient of friction, is not attacked by any known solvents, deforms at relatively high temperatures for plastics (above 330.degree. C.) and thus is a good non-stickable material.
The problem with PTFE is its cost, therefore it is not feasible to make a solid shaft or tube out of PTFE, instead the PTFE is deposited on the external surface of a metal shaft, preferably by baking. However, this coating process is time consuming and costly.